Conventional low density polyethylene (LDPE) has good processability, however, when used in film and/or extrusion coating application, increased melt strength is still desired. It has been discovered that such polymers can be produced using asymmetric polyenes. However, there is a need to produce such polymers under polymerization conditions with minimized reactor fouling and good reactor stability.
Polymerization processes for LDPE polymers using various branching agents, and other methods, are described in the following: U.S. Publication No. 2008/0242809, International Publication Nos. WO 2007/110127, WO 97/45465, WO 2012/057975, WO 2012/084787, WO2013/059042, WO2013/078018, WO2013/078224, and International Application No. PCT/US13/029881 (now WO 2014/003837).
It is well known, in the industry, that under sufficiently high pressures and temperatures, or in the presence of an ignition source, ethylene can decompose into carbon, methane and hydrogen. The following mechanism is described by Zimmermann et al., in “Explosive decomposition of compressed ethylene”, Chemie Ingenieur Technik (1994), 66 (10), 1386-1389: C2H4→(1+z) C+(1−z) CH4+2z H2, where z is in the range of 0 to 1, and depends on the pressure and temperature. This decomposition results in a runaway reaction, which produces very high temperatures and pressures, which could then lead to equipment damage. To avoid equipment damage during a decomposition, in practice, the reactor content is rapidly released to atmosphere through emergency relief valves, and possible treatment by vent cyclones and/or flares. Thus, ethylene decompositions are considered as highly unwanted events. Decomposition of ethylene has been studied extensively by Luft and others in the following publications: “Safety engineering studies on the explosive decomposition of compressed ethylene”, Chemie Ingenieur Technik (1995), 67 (7), 862-864, “Thermal decomposition of ethylene-comonomer mixtures under high pressure” AIChE Journal (1999), 45 (10), 2214-2222, and “Effect of reactor contamination on highly compressed ethylene” Chemie Ingenieur Technik (2000), 72(12), 1538-1541. Zhang et al. have also described the phenomena in “Runaway phenomena in low-density polyethylene autoclave reactors” AIChE Journal (1996), 42 (10), 2911-2925.
Every new compound introduced in low density polyethylene manufacturing technology, and which can provide additional radicals (over those from peroxides), and therefore provide the temperature needed for the above runaway reaction, needs to be tested for decomposition sensitivity. The propensity for each compound to shift the baseline level of radicals in the process must be considered. In some cases, a given compound may generate radicals, independent of other materials injected into the reactor. In other cases, an interaction between two compounds may generate additional radicals.
The interaction of aldehydes and vinyl monomers, particularly methyl methacrylate, to generate radicals is described by Ouchi et al., in “Vinyl Polymerization. 393. Radical Polymerization of Vinyl Monomer Initiated by Aliphatic Aldehyde” Bull. Chem. Soc. Jpn., 53, 748-752 (1980). Other compounds, such as ketones, for example cyclohexanone, behave similarly as described by Liu et al., in “Computational Study of Cyclohexanone—Monomer Co-initiation Mechanism in Thermal Homo-polymerization of Methyl Acrylate and Methyl Methacrylate,” J. Phys. Chem. A, 116, 5337-5348 (2012).
Fouling is an important consideration, in the use of tubular reactors for the production of high pressure, low density polyethylene. Fouling can impact capability for heating ethylene, prior to initiating reaction, or impact capability for heat removal. Oxygen (O2) is a known initiator in the high pressure, low density polyethylene process. Methacrylate or acrylate based comonomers, such as ethyl acrylate, methylacrylic acid, or acrylic acid, are often stabilized with inhibitors which require oxygen (O2) to function properly. For high pressure, low density polyethylene processes, which use methacrylate or acrylate based comonomers, stability of the comonomers must be managed, to ensure they will not initiate polymerization, prematurely, before they are injected into, or arrive in, a reactor zone. For a tubular polymerization, it is common practice that the ethylene feed, or a portion of the overall ethylene feed, to the reactor, is heated prior to the injection and/or activation of the initiator in the first reaction zone. Another area of concern is the secondary compressor, in which reactants are compressed to the inlet pressure of the reactor system. Typically inlet pressure for high pressure polyethylene reactor systems ranges from 1000 to 5000 bar. The secondary compressors are typically equipped with reciprocating plungers in single or multi stage compression configuration. In each adiabatic compression stage, the ethylene is heated up, and needs to be cooled down, before the next and/or final compression stage. This thermal effect could lead to premature formation of radicals, and consequently, polymer in the cylinder packing and/or valves, leading potentially to disturbed lubrication in the cylinder packing and/or higher cylinder discharge temperatures, by recompression of the ethylene due to leaking cylinder valves.
As discussed above, there remains a need for new ethylene-based polymers that have higher melt strengths and higher densities, and which can be polymerized with minimal reactor fouling and good process and reactor stability. These needs have been met by the following invention.